204 EXAMINATIONS OF WATER, AIR, AND SOIL. 



the solid particles of earth interfere considerably with the 

 counting of the colonies, and to obviate this it is necessary at 

 times to dissolve the soil in a certain quantity of sterile 

 water, and to make plate cultures from this water. 



It should always be remembered, however, in making 

 examinations of the soil, that a number of the bacteria found 

 in it are anaerobics, and should be cultivated as such. 



Soil taken near the surface is always rich in bacteria, and 

 the further down the investigator proceeds the smaller is the 

 number of bacteria found, until at a distance of a meter and 

 a half from the surface all bacteria have disappeared. 



In examinations of excavations made in the city of New 

 Orleans some three years ago the author had occasion to 

 verify the foregoing fact fully ; cultures made from mud at 

 different depths showed a constant diminution of micro- 

 organisms, until at a depth of between five and six feet 

 no bacteria could be obtained. 



QUESTIONS. 



What sort of bacteriological examinations of water are made? 



What is meant by a quantitative test ? 



Why is this not a fair test? 



How is a quantitative analysis of water made ? 



What instrument is useful for that purpose ? 



How should water be collected ? 



What dilutions should be used ? 



How is this dilution made ? 



How are the colonies on plates counted? 



Describe Wolfhuegel's apparatus for counting colonies on plates? 



How are Petri dishes used ? 



Describe Fakes' apparatus for counting colonies in Petri dishes. 



How are Esmarch tubes made ? 



For what bacteria is the qualitative analysis of water made ? 



What are the difficulties in making qualitative analysis? 



How is examination of water for the cholera germ made ? For typhoid 

 fever germ ? 



Describe Eisner's method of examining water for typhoid fever bacilli. 



What influence has the addition of antiseptics to water for bacterial exam- 

 ination? 



How is water to be examined for the presence of Bacillus coli communis ? 



What is the significance of the Bacillus coli communis in water? 



Describe a method for the bacteriological examination of air. 



Describe Sedgwick-Tucker's method. 



How is an examination of the soil made ? 



What bacteria are found in the soil ? 



At what depth from the surface is the soil free from bacteria ? 



